1. Field of the Invention
This invention relates to novel copolymers exhibiting electrochromism and methods for the production thereof. This invention further relates to a coloring-discoloring material resorting to electrochromism, which is particularly usable in an electrochromic display device.
2. Description of the Prior Art
The term "electrochromism" refers to the phenomenon of a reversible change in color or the transmittance of light caused by the oxidation-reduction reaction occurring on the surface or in the neighborhood of an electrode in response to the application of voltage thereto, for example. Various materials have been reported as electrochromic substances which exhibit this phenomenon of electrochromism.
The heretofore known coloring materials capable of inducing electrochromism include those which use metal oxides like tungsten oxide as disclosed in Japanese Patent Publication No. SHO 52-46098, those which use organic low-molecular weight compounds like viologen in the form of a solution as disclosed in published Japanese Patent Application, KOKAI (Early Publication) No. SHO 51-146253, and those which use electroconductive macromolecular compounds having conjugated double bonds in the main chain thereof as disclosed in published Japanese Patent Application, KOKAI No. SHO 56-67881, for example.
Of the conventional coloring materials resorting to electrochromism, the metal oxides are produced by a vacuum process such as sputtering as disclosed in published Japanese Patent Application, KOKAI No. SHO 54-41756 and, therefore, have a problem of difficulty encountered in coating a large surface area with the metal oxide. In the case of the organic low-molecular weight compounds, the production of an electrochromic display device with such an organic low-molecular weight compound, as disclosed in published Japanese Patent Application, KOKAI No. SHO 51-146253, needs interposition of a diaphragm made of a semipermeable membrane between an electrode plate and a transparent electrode plate and further presence of a solution containing an organic low-molecular weight compound like viologen and a supporting electrolyte between the transparent electrode plate and the diaphragm and, therefore, is handicapped by the inevitable complication of the electrochromic display device in construction.
Then, in the case of the electroconductive macromolecular compounds having conjugated double bonds in the main chain thereof, the polymerization of such a compound is effected directly on the surface of an electrode by the electrolytic polymerization technique, for example, as disclosed in published Japanese Patent Application, KOKAI No. SHO 60-188931. Thus, it is difficult to coat inexpensively the electrode of a large surface area with the polymer. Further, of the electroconductive macromolecular compounds having conjugated double bonds in the main chain thereof, those which are soluble in a solvent are generally non-polar and, therefore, are deficient in adhesion to an electrode which by nature has a polar surface. These electroconductive macromolecular compounds, therefore, fail to produce fast adhesion to the electrode and still suffer from a problem of layer separation.
Particularly when the production of a solid type electrochromic display device by the use of an electroconductive macromolecular compound having conjugated double bonds in the main chain thereof and a macromolecular solid electrolyte is contemplated, the adhesiveness of the macromolecular compound to the macromolecular solid electrolyte poses a problem. In the produced device, the portions of inferior adhesion inhibit ionic conduction and form defective sites in the process of coloration and discoloration. In case there the adhesion is insufficient, the produced device after protracted use develops layer separation in consequence of repetition of the cycle of generation and extinction of color, change in temperature, or the like and the portions affected by the layer separation constitute themselves defective sites in the process of coloration and discoloration.
As respects the copolymerization of the kind contemplated and disclosed by the present invention, a polymer having a thiophene oligomer in the side chain thereof is disclosed in published Japanese Patent Application, KOKAI No. HEI 4-108784. Though this polymer is notably improved in solubility, it lacks polarity and still suffers from a problem of inferior adhesion.